Model's difficult question (5)
I believe the answer is:
poser
This clue is a double definition.
'model's' is the first definition.
(synonyms)
'difficult question' is the second definition.
(I've seen this before)
(Other definitions for poser that I've seen before include "One pretends to be something they are not" , "Tricky clue" , "Particularly baffling problem" , "Spore raises a difficult question" , "Attitudiniser" .)